The scotch yoke has been used for many years as a means for converting reciprocating linear movement to rotary motion and vice versa. It has found application in a variety of machines, such as motors, pumps, and compressors which utilize a piston articulated within a closed cylinder (see, e.g., U.S. Pat. Nos. 283,558; 813,736; 999,220; and 2,628,602), as well as in compactors, pumps, punch presses, robots, sewing machines, generators, and material handlers.
The essential components of a scotch yoke are a crankpin rotated about a crankshaft center at an axial offset and a shuttle having a slot therein through which the crankpin is positioned. The motion of the shuttle is constrained to a linear path by a guide, frequently, a pair of opposing parallel guide surfaces. The crankshaft and crankpin move in rotary motion and may be either the driven elements or the driving elements. The shuttle moves in rectilinear motion and likewise may be the driven element or the driving element. Thus, the scotch yoke provides a means for converting linear to rotary motion and vice versa.
The slot within the shuttle must be at least as wide as the crankpin diameter and long enough to accommodate the crankpin dimension and its travel. A pair of competing objectives in the design of scotch yokes is to eliminate friction, as well as clearance, at the crankpin/slot interface. Friction results in energy loss in the conversion from linear to rotary motion or vice versa and also in wear of the scotch yoke. Clearance at the interface results in a loss of motion translation, commonly called "backlash", when converting from rotary to linear motion and vice versa (i.e., there is no translation during traversal of the clearance gap), and in brinelling, spalling and vibrations when the unrestrained driving element accelerates across the clearance gap and collides into the driven element. As has been recognized for many years, the consequences of clearance and friction at the slot/crankpin interface are energy inefficiency and excessive wear and tear.
U.S. Pat. No. 4,685,342 to Douglas C. Brackett, the inventor herein, discloses a scotch yoke device having a pair of opposing, offset bearing surfaces, one on either side of the crankpin slot in the shuttle. A corresponding pair of roller bearings are arranged on the crankpin coaxially and laterally displaced from one another such that each aligns with one of the pair of opposing, offset bearing surfaces of the slot within which the crankpin is positioned. While the device disclosed in the Brackett '342 Patent minimizes clearance at the crankpin/slot interface to that attributable to manufacturing tolerances and also reduces friction between the crankpin and the shuttle slot to the rolling friction of a roller bearing, these advantages are realized without providing the device with means adapted to minimize backlash which may be present at each end of the range of motion of the roller bearings relative to their associated bearing surfaces.